TOPIC: General Configuration Errors

I run several different modules and plug-ins. I've just purchased Jom Classifieds and when I attempt to save anything in General Configuration I get the following error:

403 Forbidden
You don't have permission to access /administrator/index.php on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

From what I can see all permissions are correct for this path/file. No other modules are having issues - have you seen this before or can you suggest anything? I really need to be able to change these settings